BREEDING POULTRY FOR EGG PRODUCTION. 147 



which belong to strains having large beans as the type. Hav- 

 ing isolated from the population one of these component strains 

 which breeds true to a definite type no amount of further selec- 

 tion will modify that strain. In other words Johannsen showed 

 that, in beans at least, selection is only effective to isolate or 

 pick out what heritable variations were already present as com- 

 ponents of the population to begin with. Selection within a 

 line or strain is ineffective. 



These and other results of recent work (particularly that 

 along Mendelian lines) lead to a new conception of the mechan- 

 ism of heredity which differs markedly from older views. The 

 keynote to this conception is that it is the germ cell (egg or 

 sperm) and not the body or soma which is the factor of primary 

 importance in inheritance. What the individual is like in re- 

 spect to its personal, somatic * characters is not determined by 

 the somatic characters of its parents, but by the composition or 

 constitution of the parental gametes. Thus the size of a bean 

 is determined not by the sise of its parent bean, but by the 

 gametic constitution of the latter. 



Experimental breeding along Mendelian lines has shown very 

 clearly that many characters of organisms are inherited as sepa- 

 rate units, so that by proper cross-breeding new combinations 

 of characters may be made. Thus, for example, suppose one 

 crosses together a Barred Rock, which- is a barred bird with a 

 single comb, and a Cornish Indian Game, which is a non-barred 

 bird with a pea comb. In the second generation he will have 

 barred birds with pea combs, barred birds with single combs, 

 non-barred birds with pea combs and non-barred birds with 

 single combs occurring in certain definite proportions to each 

 other. This result shows beyond question that, whatever the 

 mechanism, comb form is inherited separately from plumage 

 pattern. These characters behave as separate units. 



* For the reader not familiar with the technical terminology of biology, 

 it may be said that "somatic" is used in designation of those characters 

 of the organism which pertain to all parts except the reproductive or 

 germ-cells. These reproductive cells are called "gametes." We then have 

 the adjective "gametic," meaning "pertaining to the germ cells," in con- 

 trast to "somatic" meaning "pertaining to any or all parts of the organ- 

 ism other than the germ cells." 



